It is commonly thought that short stroke equals high rpms. What is not commonly understood is the reason for this thinking. With a longer stroke, an engine will reach it's critical piston speed at a lower rpms. If you try to twist a long stroke engine at high rpms, reliability will suffer due to the high piston speeds. But decreasing the stroke will not automatically make the engine turn higher rpms. The rpms an engine turns is relative to the port size and timing, the exhaust and carb, and the load.

These "conversion" engines, are not really rpm limited by their stroke practically speaking. If the port timing and size is increased, along with the potential flow of the muffler and carb, and you use a smaller prop, they will turn high rpms. I have a Homelite 30cc that reliably turns 9400 rpms, and the boat guys turn these Homeys at well over 10,000. The Ryobi is a different case, as it's stamped steel rod is not intended for such high rpm operation, but most of these conversion engines will support quite high rpms.

However, we're missing something important here. It is much more efficient to turn a large prop slowly than to turn a small prop at high rpms. That's why we get such great thrust, or pull, from these motors; their ability to swing large props due to their inherent torque characteristics.

The original Ultralight airplanes had small engines turning small props at high rpms and were very inefficient. The ultralight airplane movement really "took off" (sorry), when the manufacturers developed reduction drives that enabled the use of large diameter props that created much more thrust and performance. Another example of this is the antique aircraft. It takes about a 200 horsepower modern engine to equal the performance of an antique engine of only 80 horsepower. Why? Because the antique engines swung a huge 8 foot diameter prop at only 1000 to 1200 rpms. Much more efficient.

AV8TOR